The thing is DOS vanilla doesn't need dx11, and I really doubt they have recoded everything from scratch.
The changes to the enhanced edition improve game performance greatly, and it now requires a DX11-compatible video card.
And to put it bluntly, Larian is NOT going to revert the engine back to DX 9/10-compatability even though some people aren't able to play the EE on their current system. The EE also no longer supports Windows XP even though the vanilla game did.
